Hayden

Ever had a large-scale project where the build took so long you could grab coffee and it still wasn’t done? When I first built TensorFlow from source, I wondered when it would ever finish. Turns out, the tool that builds TensorFlow is exactly what we’re covering today—Bazel.

Bazel is Google’s internal build system Blaze, open-sourced in 2015. It’s designed to handle massive projects with hundreds of thousands of source files quickly and reliably. As of October 2025, Bazel 7.6.2 and 8.2 are actively maintained, with Bazel 9.0 LTS scheduled for release later this year.

 

Bazel

 

 

1. What Makes Bazel Special?

Bazel is an open-source build and test tool similar to Make, Maven, and Gradle. But it’s not just another build tool—it’s the culmination of years of Google’s experience managing massive codebases.

Bazel automates software builds and tests by running compilers and linkers to produce executable programs and libraries, and assembling deployment packages for various target environments including Android and iOS.

The Name Origin

The early logo featured the letter ‘b’ represented by two basil leaves, playing on both the internal codename “Blaze” and the herb “basil.” It’s typically pronounced “bay-zel” in English.

 

 

2. How Does Bazel Differ from Other Build Tools?

If you’re already using Maven or Gradle, you might wonder if learning another build tool is worth it. Here’s what sets Bazel apart.

Speed and Efficiency

Bazel caches all previously completed work and tracks changes to both file content and build commands. This means Bazel knows exactly what needs rebuilding and only rebuilds that.

Simply put, if you modify 1 out of 100 files, Bazel only rebuilds that file and its dependencies. Think of it as having an intelligent assistant telling you, “This part is already done, just redo this one thing.”

Multi-Platform, Multi-Language Support

Bazel runs on Linux, macOS, and Windows, building binaries and deployment packages for multiple platforms—desktop, server, and mobile—from the same project.

Python and Scala are now officially supported target languages. Beyond Java and C++, Bazel supports Python, Scala, Go, Rust, and more. It can even build Android and iOS apps.

Scalability

Bazel maintains agility while handling builds with 100,000+ source files, working with multiple repositories and tens of thousands of users.

It scales from small projects to Google-scale monorepos.

 

 

3. Understanding Bazel’s Core Concepts

To use Bazel effectively, you need to understand a few key concepts.

Workspace

A workspace is a directory where Bazel stores source code and build outputs. A directory becomes a workspace when it contains a WORKSPACE or MODULE.bazel file at the root.

The WORKSPACE file marks the project root. It can be empty or define external dependencies.

As of March 2025, the Bazel Central Registry hosts over 650 modules, with Bzlmod becoming the standard external dependency system since Bazel 7. In Bazel 9, WORKSPACE functionality will be completely removed, making Bzlmod the only way to manage dependencies.

Using MODULE.bazel for dependency management is now the standard in modern Bazel.

BUILD Files vs BUILD.bazel Files

Each package needs a file defining build rules, either BUILD or BUILD.bazel.

  • BUILD: Traditional name
  • BUILD.bazel: Alternative to avoid conflicts with system folders on Windows

Windows users should prefer BUILD.bazel. Bazel recognizes both, prioritizing BUILD.bazel when both exist in the same directory.

Here’s a simple C++ project BUILD file:

cc_binary(
    name = "hello-world",
    srcs = ["hello-world.cc"],
)

Here, cc_binary is a build rule, and hello-world is the target name.

Rules and Targets

  • Rule: Defines how to perform build tasks, like cc_binary, cc_library, java_binary, py_library.
  • Target: An instance of a rule in a BUILD file. In the example above, hello-world is a target.

Targets are referenced as //package/path:target-name. For example, //main:hello-world refers to the hello-world target in the main directory.

Starlark Language

BUILD files use Starlark, a domain-specific language with Python-like syntax, making it familiar for Python developers.

 

 

4. Installing Bazel – Step-by-Step Guide

Let’s install Bazel. The easiest and recommended method is using Bazelisk.

What is Bazelisk?

Bazelisk is a Bazel wrapper written in Go that automatically selects the appropriate Bazel version for your current directory, downloads it from the official server if needed, and transparently passes all command-line arguments to the actual Bazel binary.

Think of it as a version manager for Bazel, similar to nvm for Node.js.

macOS Installation

brew install bazelisk

One command with Homebrew.

Windows Installation

Choose your preferred method:

# Windows Package Manager
winget install Bazel.Bazelisk

# Chocolatey
choco install bazelisk

# Scoop
scoop install bazelisk

All methods add both bazelisk and bazel to your PATH.

Windows User Notes:

  • Avoid spaces in paths. Use C:\bazel-workspace instead of C:\Program Files.
  • You may need to enable Long Path support. Run PowerShell as administrator: 
    New-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Control\FileSystem" -Name "LongPathsEnabled" -Value 1 -PropertyType DWORD -Force

Linux Installation

Download the binary directly:

# Download latest Bazelisk
wget https://github.com/bazelbuild/bazelisk/releases/latest/download/bazelisk-linux-amd64

# Make executable
chmod +x bazelisk-linux-amd64

# Move to PATH location
sudo mv bazelisk-linux-amd64 /usr/local/bin/bazel

# Verify installation
which bazel

npm Installation

Frontend developers can use npm:

npm install -g @bazel/bazelisk

Verify Installation

bazel --version

If you see version information, you’re good to go!

 

 

5. Your First Bazel Project

Let’s build a simple C++ project to see Bazel in action.

Create Project Structure

mkdir my-bazel-project
cd my-bazel-project

Create WORKSPACE File

Create an empty WORKSPACE file at the project root:

# Linux/macOS
touch WORKSPACE

# Windows (PowerShell)
New-Item -Path . -Name "WORKSPACE" -ItemType "file"

For modern Bazel, MODULE.bazel is preferred, but an empty WORKSPACE suffices for this basic example.

Write Source Code

Create hello-world.cc:

#include <iostream>

int main() {
    std::cout << "Hello, Bazel World!" << std::endl;
    return 0;
}

Create BUILD File

In the same directory, create a BUILD file (or BUILD.bazel on Windows):

cc_binary(
    name = "hello-world",
    srcs = ["hello-world.cc"],
)

Run the Build

bazel build //:hello-world

// refers to the workspace root, and :hello-world is the target name.

A successful build shows:

INFO: Analyzed target //:hello-world (15 packages loaded, 58 targets configured).
INFO: Found 1 target...
Target //:hello-world up-to-date:
  bazel-bin/hello-world
INFO: Elapsed time: 2.456s, Critical Path: 0.89s
INFO: Build completed successfully, 3 total actions

Run It

bazel run //:hello-world

Or run the built binary directly:

# Linux/macOS
./bazel-bin/hello-world

# Windows
.\bazel-bin\hello-world.exe

If you see “Hello, Bazel World!”, success! 🎉

 

 

6. A More Practical Example – Library Dependencies

Real projects involve multiple files with dependencies. Let’s create a library and use it.

Project Structure

my-project/
├── WORKSPACE
├── lib/
│   ├── BUILD
│   ├── greet.h
│   └── greet.cc
└── main/
    ├── BUILD
    └── main.cc

Write Library Code

lib/greet.h:

#ifndef GREET_H
#define GREET_H

#include <string>

std::string get_greeting(const std::string& name);

#endif

lib/greet.cc:

#include "lib/greet.h"

std::string get_greeting(const std::string& name) {
    return "Hello, " + name + "!";
}

lib/BUILD:

cc_library(
    name = "greet",
    srcs = ["greet.cc"],
    hdrs = ["greet.h"],
    visibility = ["//visibility:public"],
)

The visibility attribute makes this library accessible from other packages.

Write Main Program

main/main.cc:

#include <iostream>
#include "lib/greet.h"

int main() {
    std::cout << get_greeting("Bazel") << std::endl;
    return 0;
}

main/BUILD:

cc_binary(
    name = "hello",
    srcs = ["main.cc"],
    deps = ["//lib:greet"],
)

The deps attribute specifies //lib:greet as a dependency.

Build and Run

bazel build //main:hello
bazel run //main:hello

Output: “Hello, Bazel!”

Visualize Dependencies

One of Bazel’s powerful features is dependency visualization:

bazel query --notool_deps --noimplicit_deps 'deps(//main:hello)' --output graph

This outputs a dependency graph in GraphViz format. Paste it into Graphviz Online to visualize.

 

 

7. Bazel’s Powerful Features

Incremental Builds

When you modify just one file, Bazel doesn’t rebuild everything. If you modify only greet.cc:

bazel build //main:hello

Bazel recompiles only greet.cc and dependent files, using cache for the rest.

Parallel Builds

Bazel processes multiple tasks in parallel by default, efficiently utilizing CPU cores to significantly reduce build times.

Control parallel jobs:

bazel build //main:hello --jobs=8

Remote Cache

Teams can share build caches. When someone builds something, others can reuse those results—especially useful in CI/CD environments.

Test Automation

cc_test(
    name = "greet_test",
    srcs = ["greet_test.cc"],
    deps = ["//lib:greet"],
)

Manage tests with Bazel:

bazel test //lib:greet_test

 

 

8. Managing Options with .bazelrc Configuration

Typing options repeatedly is tedious. Create a .bazelrc file to store default settings.

Create .bazelrc in your project root:

# Build optimization
build --jobs=8
build --compilation_mode=opt

# Test settings
test --test_output=errors

# Windows-specific
build:windows --enable_runfiles=false
build:windows --copt=-DWIN32_LEAN_AND_MEAN

# macOS-specific
build:macos --macos_minimum_os=10.15

Use specific configurations:

bazel build //main:hello --config=windows

 

 

9. IDE Integration – IntelliJ IDEA Example

The Bazel plugin for IntelliJ IDEA 2025.1 has seen major improvements, with full support for Bazel 8 and nested modules, plus official Python and Scala support.

Setting Up IntelliJ IDEA

  1. Launch IntelliJ IDEA
  2. Go to Settings > Plugins
  3. Search for “Bazel” in Marketplace and install
  4. Restart the IDE

Opening a Project

  1. File > Open and select the directory containing the WORKSPACE file
  2. Select “Import Bazel Project”
  3. The IDE analyzes the project structure and provides code assistance

You can add dependencies directly from code. When using a class from another part of your Bazel project that isn’t yet a dependency, IntelliJ’s standard Alt+Enter intention actions offer an “Add dependency on module…” option.

Convenient, right? No manual BUILD file editing needed.

 

 

10. Troubleshooting – Common Issues and Solutions

Issue 1: “ERROR: no such package”

Symptom:

ERROR: no such package '//lib': BUILD file not found in directory 'lib'

Cause: Missing or misplaced BUILD file.

Solution:

  • Verify BUILD or BUILD.bazel file exists in lib directory
  • Check filename case sensitivity (Linux/macOS are case-sensitive)

Issue 2: “ERROR: Target not found”

Symptom:

ERROR: no such target '//main:helo': target 'helo' not declared

Cause: Typo in target name or undeclared in BUILD file.

Solution:

  • Check name attribute in BUILD file
  • Verify spelling

Issue 3: Windows Path Issues

Symptom:

ERROR: error reading BUILD file

Cause: Windows path separator issues

Solution:

  • Always use forward slashes (/) in BUILD files
  • Use slashes (/) instead of backslashes (\)

Issue 4: Cache Problems

Symptom: Code changes not reflected after modification

Solution:

# Clear cache
bazel clean

# Complete cleanup (removes all build outputs)
bazel clean --expunge

Issue 5: Out of Memory

Symptom:

ERROR: Java heap space

Solution: Add memory settings to .bazelrc

startup --host_jvm_args=-Xmx4g

 

 

11. Practical Tips and Best Practices

Using .bazelversion File

Create a .bazelversion file in your project root to specify the exact Bazel version:

7.6.2

This ensures the entire team uses the same version, preventing “works on my machine” issues.

Viewing Build Logs

For detailed logs when builds fail:

bazel build //main:hello --verbose_failures

For even more detail:

bazel build //main:hello --subcommands

Performance Profiling

To analyze build speed:

bazel build //main:hello --profile=build.prof

Then analyze results:

bazel analyze-profile build.prof

Testing Specific Targets

Test specific packages instead of everything:

# All tests in lib package
bazel test //lib/...

# Specific test only
bazel test //lib:greet_test

Finding Build Outputs

Bazel creates symbolic links for easy output access:

  • bazel-bin/: Binary outputs
  • bazel-out/: All build outputs
  • bazel-testlogs/: Test logs

 

 

12. Major Projects Using Bazel

If you doubt Bazel’s power, consider these projects using it:

  • TensorFlow: Google’s machine learning framework
  • Kubernetes: Container orchestration platform
  • Envoy: High-performance proxy
  • gRPC: Google’s RPC framework
  • Angular: Google’s web framework

These large-scale projects chose Bazel for clear reasons: speed, reliability, and scalability.

 

 

13. Bazel vs Other Build Tools

Bazel vs Maven

Feature Bazel Maven
Speed Very fast (caching, parallel) Moderate
Learning Curve Steep Gentle
Language Support Multi-language Mainly Java
Caching Local + remote Local only
Ecosystem Growing Very mature

Bazel vs Gradle

Feature Bazel Gradle
Build Speed Very fast Fast
Incremental Builds Very precise Good
Platform Support Multi-platform Mainly JVM
Configuration Explicit Flexible
Android Dev Supported Official tool

When to Choose Bazel

Bazel is ideal for:

  • Managing large-scale monorepos
  • Multi-language, multi-platform projects
  • Speed-critical builds
  • Large teams requiring build reproducibility

Other tools may be better for:

  • Small single-language projects
  • Rapid prototyping
  • Heavy dependence on existing ecosystems

 

 

14. 2025 Bazel Roadmap and Community

Bazel 9.0 LTS is scheduled for late 2025, completely removing WORKSPACE functionality in favor of Bzlmod only.

Notable items include WORKSPACE deprecation, Starlarkification of C++ rules and autoload removal, lazy evaluation of symbolic macros, and a new project-based model to reduce cognitive burden from Bazel flags.

BazelCon 2025

The Bazel community’s annual event, BazelCon 2025, will be held November 10-11, 2025 in Atlanta, Georgia, USA. A Community Training Day is scheduled for November 9.

Attendance is free, and you can register at the official site. It’s a great opportunity to connect with the Bazel team and hear the latest announcements.

The Bazel team continuously improves developer experience, and the community remains vibrant.

 

 

 

What started as “why learn another build tool?” has turned into “I can’t imagine large-scale projects without Bazel.” Especially for projects mixing multiple languages or where build time matters, Bazel makes a real difference.

The learning curve is steep, and migrating existing projects to Bazel isn’t trivial. But once properly set up, the investment pays off.

I hope this guide helps you get started with Bazel. For questions, check the official Bazel documentation and GitHub repository. The community is active and helpful.

For more information:

 

 

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